全文获取类型
收费全文 | 24499篇 |
免费 | 4081篇 |
国内免费 | 4027篇 |
专业分类
化学 | 19245篇 |
晶体学 | 356篇 |
力学 | 1222篇 |
综合类 | 281篇 |
数学 | 2464篇 |
物理学 | 9039篇 |
出版年
2024年 | 52篇 |
2023年 | 456篇 |
2022年 | 653篇 |
2021年 | 856篇 |
2020年 | 972篇 |
2019年 | 979篇 |
2018年 | 801篇 |
2017年 | 863篇 |
2016年 | 1204篇 |
2015年 | 1222篇 |
2014年 | 1601篇 |
2013年 | 1957篇 |
2012年 | 2378篇 |
2011年 | 2485篇 |
2010年 | 1801篇 |
2009年 | 1674篇 |
2008年 | 1862篇 |
2007年 | 1640篇 |
2006年 | 1467篇 |
2005年 | 1210篇 |
2004年 | 870篇 |
2003年 | 737篇 |
2002年 | 795篇 |
2001年 | 690篇 |
2000年 | 520篇 |
1999年 | 476篇 |
1998年 | 349篇 |
1997年 | 296篇 |
1996年 | 256篇 |
1995年 | 218篇 |
1994年 | 213篇 |
1993年 | 179篇 |
1992年 | 169篇 |
1991年 | 138篇 |
1990年 | 101篇 |
1989年 | 113篇 |
1988年 | 60篇 |
1987年 | 50篇 |
1986年 | 59篇 |
1985年 | 39篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 17篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1974年 | 10篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
41.
42.
Solitary waves and their bifurcations of KdV like equation with higher order nonlinearity 总被引:3,自引:1,他引:2
We investigate the KdV like equation with higher order nonlinearity ut + a(1 +bun)unux + uxxx = 0with n ≥ 1, a, b ∈ R and α≠ 0. The bifurcations and explicit expressions of solitary wave solutions for theequation are discussed by using the bifurcation method and qualitative theory of dynamical systems. Thebifurcation diagrams, existence and number of the solitary waves are given. 相似文献
43.
44.
45.
利用解析和数值方法研究了在具有横向折射率周期性调制的克尔型非线性介质中光学格子孤子的传输,得到了孤子参数的演化方程以及格子孤子的形成和稳定传输的条件.结果表明:当光束的入射角小于某临界角度时,光束可被类似波导形式的路径俘获而稳定传输,该临界角随折射率调制周期、调制深度的增加而增大,且光束越窄临界值越大.此外,线性空间啁啾虽然对光束传输的中心位置没有任何影响,但会导致光束发散从而破坏格子孤子的形成和稳定传输,对此提出了采用特定功率取值来补偿啁啾作用从而形成格子孤子的方案.
关键词:
光孤子
光学格子
光传输
矩方法 相似文献
46.
Parallel to Cox's [JRSS B34 (1972) 187-230] proportional hazards model, generalized logistic models have been discussed by Anderson [Bull. Int. Statist. Inst. 48 (1979) 35-53] and others. The essential assumption is that the two densities ratio has a known parametric form. A nice property of this model is that it naturally relates to the logistic regression model for categorical data. In astronomic, demographic, epidemiological, and other studies the variable of interest is often truncated by an associated variable. This paper studies generalized logistic models for the two-sample truncated data problem, where the two lifetime densities ratio is assumed to have the form exp{α+φ(x;β)}. Here φ is a known function of x and β, and the baseline density is unspecified. We develop a semiparametric maximum likelihood method for the case where the two samples have a common truncation distribution. It is shown that inferences for β do not depend the nonparametric components. We also derive an iterative algorithm to maximize the semiparametric likelihood for the general case where different truncation distributions are allowed. We further discuss how to check goodness of fit of the generalized logistic model. The developed methods are illustrated and evaluated using both simulated and real data. 相似文献
47.
Orlicz-Sobolev空间的中点局部一致凸性 总被引:2,自引:0,他引:2
本文研究了Orlicz-Sobolev空间的中点局部一致凸性,通过结合Orlicz空间和Sobolev空间的技巧得到分别赋Luxemburg范数和赋Orlicz范数的Orlicz-Sobolev空间具有中点局部一致凸性的充要条件. 相似文献
48.
49.
The lattice profile analyzes the intrinsic structure of pseudorandom number sequences with applications in Monte Carlo methods and cryptology. In this paper, using the discrete Fourier transform for periodic sequences and the relation between the lattice profile and the linear complexity, we give general formulas for the expected value, variance, and counting function of the lattice profile of periodic sequences with fixed period. Moreover, we determine in a more explicit form the expected value, variance, and counting function of the lattice profile of periodic sequences for special values of the period. 相似文献
50.
Based on the rate equations and the power propagation equations, the amplified model is obtained which considers the cross-phase modulation (XPM)-induced intensity fluctuation between pump power and signal power in erbium-doped fiber amplifier (EDFA). Simulation results show: a higher nonlinear coefficient and a stronger pump power cause a stronger XPM in EDFA. Before the optimal length of EDF, the intensity caused by XPM increases with the increasing length, but after the optimal length, the XPM presents saturation. A bigger nonlinear coefficient γ corresponds to a smaller optimal length and gain, and makes gain saturation more easy. XPM effect will decrease EDFA’s amplified efficiency. 相似文献